逐次近似再構成法を応用した新しい画像再構成法に対する画質評価

摘要

Computed tomography（CT）装置では従来，画像再構成法としてfiltered back projection（FBP）が用いられてきた．この手法では実空間上の投影によって一次元投影データを得た後，特定の周波数成分を強調もしくは減弱させるフィルタを適用し，連投影することで投影物体を復元することが可能である．この手法は，高速な画像再構成演算が可能となる一方で，データの矛盾に伴うさまざまなアーチファクトやX線の統計的な揺らぎによるノイズの影響がそのまま反映され，それを補正するには投影データに対する補正処理や再構成画像の後処理が必要となる．%The purpose of this study was to evaluate the image quality of an iterative reconstruction method, the iterative reconstruction in image space (IRIS), which was implemented in a 128-slices multi-detector computed tomography system (MDCT), Siemens Somatom Definition Flash (Definition). We evaluated image noise by standard deviation (SD) as many researchers did before, and in addition, we measured modulation transfer function (MTF), noise power spectrum (NPS), and perceptual low-contrast detectability using a water phantom including a low-contrast object with a 10 Hounsfield unit (HU) contrast, to evaluate whether the noise reduction of IRIS was effective. The SD and NPS were measured from the images of a water phantom. The MTF was measured from images of a thin metal wire and a bar pattern phantom with the bar contrast of 125 HU. The NPS of IRIS was lower than that of filtered back projection (FBP) at middle and high frequency regions. The SD values were reduced by 21%. The MTF of IRIS and FBP measured by the wire phantom coincided precisely. However, for the bar pattern phantom, the MTF values of IRIS at 0.625 and 0.833 cycle/ mm were lower than those of FBP. Despite the reduction of the SD and the NPS, the low-contrast detectability study indicated no significant difference between IRIS and FBP. From these results, it was demonstrated that IRIS had the noise reduction performance with exact preservation for high contrast resolution and slight degradation of middle contrast resolution, and could slightly improve the low contrast detectability but with no significance.